Write the drawbacks in Mendeleev's periodic table that led to its modification.
Write the drawbacks in Mendeleev's periodic table that led to its modification.
Drawbacks of Mendeleev’s periodic table are discussed below:
1. Position of hydrogen:
Hydrogen is placed in group I. However, it resembles the elements of group I (alkali metals) as well as the elements of group VIIA, (halogens). Therefore, the position of hydrogen in the periodic table is not correctly defined.
2.Anomalous pairs:
In certain pairs of elements, the increasing order of atomic masses was not obeyed. In these cases, Mendeleev placed elements according to similarities in their properties and not in increasing order of their atomic masses. For example, argon (Ar, atomic mass39.9) is placed before potassium (K, atomic mass39.1). Similarly, cobalt (Co, atomic mass 58.9) is placed before nickel (Ni, atomic mass 58.6)
3. Position of isotopes:
Isotopes are the atoms which is having different atomic masses but same atomic number. Therefore, according to Mendeleev’s classification, these should be placed at different places depending upon their atomic masses.
4. Some similar elements are separated and dissimilar elements are grouped together:
In Mendeleev's periodic table, some similar elements were placed in different groups while some dissimilar elements had been grouped together.
For example, copper and mercury resemble each other in their properties but they have been placed in different groups.
At the same time, elements of group IA such as Li,Na and K were grouped with copper (Cu), silver (Ag) and gold (Au), though their properties are quite different.
5.Cause of periodicity:
Mendeleev did not explain the cause of periodicity among the elements.
6. Position of lanthanides (or lanthanides) and actinoids (or actinides):
The fourteen elements following lanthanum (known as lanthanoids, from atomic number58−71) and the fourteen elements following actinium (known as actinoids, from atomic number90–103) have not been given separate places in Mendeleev’s table.
In order to cover more elements, Mendeleev modified his periodic table.
Drawbacks of Mendeleev’s periodic table are discussed below:
1. Position of hydrogen:
Hydrogen is placed in group I. However, it resembles the elements of group I (alkali metals) as well as the elements of group VIIA, (halogens). Therefore, the position of hydrogen in the periodic table is not correctly defined.
2.Anomalous pairs:
In certain pairs of elements, the increasing order of atomic masses was not obeyed. In these cases, Mendeleev placed elements according to similarities in their properties and not in increasing order of their atomic masses. For example, argon (Ar, atomic mass39.9) is placed before potassium (K, atomic mass39.1). Similarly, cobalt (Co, atomic mass 58.9) is placed before nickel (Ni, atomic mass 58.6)
3. Position of isotopes:
Isotopes are the atoms which is having different atomic masses but same atomic number. Therefore, according to Mendeleev’s classification, these should be placed at different places depending upon their atomic masses.
4. Some similar elements are separated and dissimilar elements are grouped together:
In Mendeleev's periodic table, some similar elements were placed in different groups while some dissimilar elements had been grouped together.
For example, copper and mercury resemble each other in their properties but they have been placed in different groups.
At the same time, elements of group IA such as Li,Na and K were grouped with copper (Cu), silver (Ag) and gold (Au), though their properties are quite different.
5.Cause of periodicity:
Mendeleev did not explain the cause of periodicity among the elements.
6. Position of lanthanides (or lanthanides) and actinoids (or actinides):
The fourteen elements following lanthanum (known as lanthanoids, from atomic number58−71) and the fourteen elements following actinium (known as actinoids, from atomic number90–103) have not been given separate places in Mendeleev’s table.
In order to cover more elements, Mendeleev modified his periodic table.
